Eight officers in black suits attempted to provide a cordon around the vehicle the pope was travelling but were overwhelmed by the sheer number of well-wishers trying to touch him, greet him or take his photograph.

At one point the pope's cavalcade was boxed in behind a line of buses and came to a standstill, allowing at least one woman to present her baby at the open car window for Francis to kiss.

Others darted around the front and back of the car, evading the security entourage and thrusting their hands through the window to touch the pontiff.

The chaotic scenes, which happened after Francis arrived in Rio from Rome at the start of a week-long trip, raised serious questions about the security arrangements for the visit.

"I've covered papal events for a quarter of a century and I've seen big crowds but I've never seen people swarming around the Pope's car like that," said Gerard O'Connell, a veteran Vatican correspondent. "The police were really struggling to control the situation." Brazilian authorities are said to be privately aghast at the Pope's decision to travel in an open-sided Mercedes jeep during his visit, rather than a glass-enclosed, bullet-proof Pope-mobile.

He will tour a city that was recently convulsed by street protests over government corruption, high transport costs and the expense of next year's World Cup and the 2016 Olympics.

More than a million Brazilians took to the streets in recent weeks.

Francis, the first Latin American pope in history, arrived in Rio after a 12 hour flight from Rome.

Pope Francis walks with Brazil's President Dilma Rousseff (REUTERS)

Crowds cheered and choirs sang when he stepped onto the tarmac of Rio's airport. He was met by president Dilma Rousseff and other senior officials.

After the chaotic scenes on roads leading into Rio, he then transferred to a white open-top jeep that drove him through central parts of the tropical city.

His security detail, including officers from the Vatican Gendarmerie, was able to re-establish control because barriers kept the crowds back along the route.

But there will be fresh security headaches in the coming days. On Thursday he will venture into one of Rio's favelas, touring the shanty town on foot, meeting local families and visiting a chapel.

The Vatican says it has been assured by the Brazilian authorities that the slum has been "pacified" and cleared of arms and drugs.

Meawhile, a homemade explosive device was discovered in the bathroom of the parking garage of the Brazilian shrine that Pope Francis will visit this week, the military said.

The military said the device was destroyed after its discovery at the Shrine of Our Lady of Aparecida between Rio and Sao Paulo.

Later on Thursday, Francis will attend a ceremony on Copacabana beach for an official welcoming ceremony by the hundreds of thousands of young people taking part in World Youth Day.

On Friday he will return to Copacabana beach, where he will walk the Stations of the Cross amid a crowd of up to a million people.

Brazilian security officials say that policing the 2.5 mile long beach is their toughest challenge during the week-long visit.

Politicians, police and army commanders had urged the pope to use an armoured Pope-mobile on the trip.

But he has insisted on a much more informal, open style, and relishes the prospect of getting close to the faithful.

"The bullet proofing would lessen our worries, it'd be better if he had it," General Jose Abreu, the officer overseeing the military's role in the security scheme, told the Associated Press. "It's a personal choice and we'll respect it, but it's not remotely pleasant for security forces."

The visit is being seen as a test of Brazil's capacity to host next year's World Cup tournament and the 2016 Olympics.